H. XVIII.] 



RADIO-ACTIVITY 



171 



Emanations. 



The discovery of thorium and radium emanations 

 was made by Rutherford and Dorn respectively in 

 consequence of an observation of Owens on the 

 irregularity of thorium rays in producing ionisation, 

 the fact being that any of these materials are more 

 active when the emanation has been allowed to 

 accumulate than soon after it has been removed. 

 For the emanation, although so infinitesimal in 

 quantity, is considerably more active than the sub- 

 stance itself; and, being a gas, it can readily be 

 drawn away or otherwise expelled from the pores or 

 neighbourhood of the salt. But it accumulates again, 

 being evidently generated in situ, and presently the 

 full activity of the substance is restored. Radium 

 emanation is shown by Rutherford and Soddy to 

 liquefy at a temperature of about 150 degrees below 

 zero; thorium emanation liquefies at about 120 C. 

 They appear to be quite definite, though transitory 

 and very unstable and disintegrating, materials. 



Deflexion of Alpha-rays. 



When alpha-rays are submitted to a strong 

 magnetic field they are deflected, though very 



